This invention relates to coatings for high temperature applications, such as gas turbine assemblies.
The design of modern gas turbines is driven by the demand for higher turbine efficiency. It is widely recognized that turbine efficiency can be increased by operating the turbine at higher temperatures. In order to assure a satisfactory life span at these higher temperatures, thermal barrier coatings (hereinafter referred to as “TBCs”) are applied to airfoils and combustion components of the turbine, such as transition pieces and combustion liners, using various techniques.
One important aspect of TBC's is their ability to tolerate strain in the underlying component without becoming detached from the component. Because TBC's are typically made of ceramic materials having much lower inherent ductility than their underlying metallic components, various microstructural features are typically incorporated into the TBC to provide it with improved strain tolerance. For instance, TBC's deposited by plasma spray processes typically incorporate significant porosity, vertical microcracks, or combinations thereof as a means to enhance the ability of the TBC to tolerate strain. TBC's deposited by vapor processes, such as physical vapor deposition (PVD), typically are fabricated under conditions that encourage nucleation and growth of discrete, tightly packed, columnar grains, which provides a compliant microstructure with a relatively high degree of strain tolerance.
Although PVD processes provide coatings with very attractive strain tolerance properties, they tend to be relatively expensive and applicable to relatively small components when compared with plasma spray processes, because PVD processes require a vacuum chamber and supporting equipment. On the other hand, traditional thermal spray processes tend to produce coatings with lower strain tolerance and substrate adhesion than PVD processes, and generally require ancillary surface preparation processes, such as grit blasting and deposition of rough bond coats, to provide adequate adhesion to the underlying component.
Therefore, there is a need for coatings with high strain tolerance, high adhesion, and reduced need for surface preparation processes, that can be applied via comparatively inexpensive and scalable processes such as plasma spray processes.